Natural tooth shows a complex gradient in translucency, beginning from translucent (enamel) to nearly opaque (dentin) from outer to inner area. Translucency is therefore an important property of dental materials where esthetic matters.
Using zirconia material for dental applications is state of the art since several years, especially in high loaded areas of the jaw. Zirconia fulfills the practitioner's demand for dental application, especially with respect to the combination of high strength and a tooth like appearance.
Nevertheless, the translucency of zirconia is often considered to be not high enough. Therefore, more translucent, glassy veneering materials are often applied to the outer surface of the zirconia dental restoration trying to imitate enamel color and translucency. Unfortunately the strength of veneering material is more than ten times lower than the strength of zirconia. Thus, in clinical cases fractures like veneer chipping are sometimes observed. Moreover, the veneering technique is typically time and cost consuming.
Recently higher translucent zirconia materials have been developed, which can be used without veneering material, i.e. monolithically. This technique usually avoids chipping, saves time, but even with this higher translucent zirconia materials, the translucency is still considered as not being high enough to fulfill the high-end aesthetic demand for a modern dental material.
Besides a high translucency, the zirconia material should be tooth-colored. In order to achieve this objective, the pre-sintered zirconia material is typically treated with coloring solutions in its pre-sintered and absorbent state. The coloring solutions typically contain coloring ions, dissolved in a solvent, which can be applied to the pre-sintered zirconia material, to achieve a tooth-like color. Incorporating coloring components, however, goes along with a further absorption of light. Thus, after sintering a colored zirconia material is less transparent than a colored zirconia material.
Thus, veneering the colored zirconia material after sintering is often still needed to achieve the desired aesthetics. Beside zirconia materials, glass-ceramic materials exist on the dental market.
However, the strength of these materials is typically three to ten times lower than the strength of zirconia materials. Further, the glass-ceramic materials are typically provided in a pre-colored state, so that individualization can only be done by applying veneering materials as described above but not by using coloring solutions. Veneering of glass-ceramic materials often leads to similar chipping problems as zirconia materials do. Thus, currently it seems that the best aesthetics can be achieved by veneering a high strength colored zirconia ceramic material with a low strength high translucent and glass-based veneering ceramic.
WO 2004/110959 relates to a coloring solution for ceramic framework. The solution comprises a solvent (e.g. water), a metal salt and polyethylene glycol having a Mn in the range of 1,000 to 200,000. WO 00/46168 A1 (corresponding to U.S. Pat. No. 6,709,694 B1) refers to coloring ceramics by way of ionic or complex-containing solutions containing defined concentrations of at least one salts or complexes of the rare earth elements or of the elements of the subgroups. The solution might contain additives like stabilizers, complex builders, pigments and beating additives.
WO 2008/098157 relates to a coloring solution for dental ceramic framework comprising a solvent, a coloring agent comprising metal ions, and a complexing agent, wherein the amount of complexing agent is sufficient to dissolve the coloring agent in the solvent.
WO 2009/014903 relates to a coloring solution for dental ceramic articles, the solution comprising a solvent and a coloring agent comprising rare earth element ions being present in the solution in an amount of at least about 0.05 mol/l solvent and transition ions being present in the solution in an amount of about 0.00001 to about 0.05 mol/l solvent. CN 102344285 relates to method how to change light transmission of dental zirconia materials. A pre-sintered Y-TZP zirconia is dipped in or brushed with an yttrium containing solution for 2 to 15 min. The zirconia is dried and sintered at 1400 to 1600° C. for 2 h.
WO 2012/125885 A1 describes a dental ceramic article comprising ceramic components, the ceramic components comprising ZrO2 and Al2O3 and at least one component comprising Mn, Er, Pr or mixtures thereof, Al2O3 being present in an amount from about 0 to about 0.15 wt.-%. The pore diameter of the ceramic article in the pre-sintered state is said to be in a range from about 10 nm to about 500 nm. The ceramic article can be produced by applying techniques like uniaxial pressing, cold isostatic pressing, rapid-prototyping and slip casting. The content of these references is herewith incorporated by reference.
The coloring solutions typically comprise water, metal cations selected from rare earth elements, transition metal and mixtures thereof, optionally complexing agent(s) and/or further additives like (poly)ethylene glycol. The coloring solutions are typically used for homogeneously coloring of porous dental ceramics.